1. Field of the Invention
The present invention relates to a variable nozzle unit and a variable geometry system turbocharger, which are capable of changing a passage area for (a flow rate of) an exhaust gas to be supplied to a turbine impeller side in a variable geometry system turbocharger.
2. Description of the Related Art
Various developments for variable nozzle units to be installed in variable geometry system turbochargers have been underway in recent years (see Japanese Patent Application Laid-open Publications Nos. 2010-65591, 2010-71138 and 2010-71142). A concrete configuration common among variable nozzles unit based on the related art is as follows.
A base ring is arranged coaxial with a turbine impeller inside a turbine housing in a variable geometry system turbocharger. Multiple support holes are penetratingly formed in the base ring at equal intervals in the circumferential direction of the base ring. Furthermore, multiple variable nozzles are arranged on the base ring at equal intervals in the circumferential direction of the base ring in a way that the variable nozzles encompass the turbine impeller. Each variable nozzle is rotatable around its axis which is in parallel with the axis of the turbine impeller. Moreover, a nozzle shaft is integrally formed on a lateral surface of each variable nozzle. Each nozzle shaft penetrates the corresponding support hole in the base ring, and is rotatably supported by the support hole.
A link mechanism configured to synchronously rotate the multiple variable nozzles in forward and reverse directions (opening and closing directions) is arranged on one side of the base ring. To put it specifically, a guide ring is provided coaxial with the base ring in a section which is a part of a bearing housing in the variable geometry system turbocharger, and which is opposed to the back surface of the turbine impeller. Furthermore, a drive ring is rotatably provided on the outer peripheral surface of the guide ring. The drive ring rotates in forward and reverse directions by being driven by a rotary actuator. As many engagement portions as the variable nozzles are provided on a lateral surface of the drive ring at equal intervals in the circumferential direction of the drive ring. In addition, a base end portion of a synchronous link member (a nozzle link member) is integrally connected to the nozzle shaft of each variable nozzle. Tip end portions (tip end-side portions) of each synchronous link member are engaged with the corresponding engagement portion of the drive ring while nipping the engagement portion.
Thus, when an engine speed is in a high speed range and a flow rate of an exhaust gas is high, the rotary actuator drives and rotates the drive ring in the forward direction. Thereby, the multiple synchronous link members swing in the forward direction, and the multiple variable nozzles synchronously rotate in the forward direction (the opening direction). Accordingly, the passage area for the exhaust gas to be supplied to the turbine impeller side increases, and a larger amount of the exhaust gas is supplied to the turbine impeller.
On the other hand, when the engine speed is in a low speed range and the flow rate of the exhaust gas is low, the rotary actuator drives and rotates the drive ring in the reverse direction. Thereby, the multiple synchronous link members swing in the reverse direction, and the multiple variable nozzles synchronously rotate in the reverse direction (the closing direction). Accordingly, the passage area for the exhaust gas to be supplied to the turbine impeller side decreases, while a flow velocity of the exhaust gas increases. As a result, the amount of work done by the turbine impeller is secured sufficiently.